1. Field of the Invention
This invention pertains generally to the field of display units that provide information to the operator of a vehicle such as the pilot or flight crew of an aircraft.
2. Description of the Related Art
The generation of a three-dimensional image of terrain in a synthetic vision system may be accomplished using OpenGL, a standard specification defining a software interface to graphics hardware. A series of processing steps known generally as an “OpenGL Pipeline” are employed to render three-dimensional image data for presentation on the two-dimensional screen of a display unit. One of the processing steps known generally as “OpenGL Transformation” receives vertex data such as three-dimensional terrain data and transforms such data into two-dimensional screen coordinates (or window coordinates) data.
OpenGL Transformation is a series of processing steps known to those skilled in the art, and only a brief summary of these steps is provided. Coordinates of an object may be derived from vertex data, where objet coordinates are measured in reference to object space. When the object coordinates are multiplied by a matrix known generally as GL_MODELVIEW, eye coordinates of the object are yielded, where eye coordinates are measured in reference to eye space. When the eye coordinates are multiplied by a matrix known generally as GL_PROJECTION which integrates clipping and normalized device coordinates (“NDC”) transformations, the eye coordinates of the object will become mapped to NDC. Then, by means of a viewport transformation, the coordinates of the NDC are scaled and translated in order to fit into the screen.
The GL_PROJECTION matrix includes a defined frustum for the purpose of performing the clipping transformation. In a perspective projection, a pyramid frustum originating from the origin of the eye coordinate system is generally employed, where the pyramid frustum has been truncated by a near plane (or projection plane). Objects and partial objects falling inside the truncated frustum will be projected onto the near plane, and those falling outside of the frustum will be clipped out.
Objects and partial objects may be comprised of, in part, objects stored in an aerodrome mapping database (“AMDB”) as described in industry standards published by Aeronautical Radio, Inc. (“ARINC”) and/or Radio Technical Commission for Aeronautics (“RTCA”) such as, but not limited to, ARINC Specification 816 entitled “Embedded Interchange Format for Airport Mapping Database” and RTCA DO-272A entitled “User Requirements for Aerodrome Mapping Information.” DO-272A provides for aerodrome surface mapping requirements for aeronautical uses particularly on-board aircraft.
Objects stored in the AMDB include, in part, data representative of airport surfaces and surface markings corresponding to surface such as, but not limited to, runways, taxiways, and aprons. When projected onto the near plane, the widths of surfaces that are perpendicular or nearly perpendicular to the surface on which the aircraft is located may appear wide or narrow, depending on their relative location to the aircraft. Surfaces relatively close to the aircraft may appear wide and surface relatively far from the aircraft may line appear narrow. When referenced to the horizon line, surfaces appearing closer to the horizon line may appear thinner than those surfaces that are not. When surface markings are superimposed against their corresponding surfaces, the relatively thin widths of surfaces superimposed by its corresponding markings may make it difficult for the pilot to discern or distinguish between a surface and surface markings for those surfaces appearing near the horizon line. This could result with pilot confusion, a loss of situational awareness, and add to the visual clutter of the display when symbology is added and superimposed against the surfaces and surface markings.